walletkit: add startheight parameter to ListSweeps

By default ListSweeps will list all sweeps known to the wallet. Over
time this may become expensive to call and callers may not be interested
in the full history. This commit adds a `startheight` parameter to the
`ListSweeps` RPC call. This parameter can be used to fetch sweeps only
from a specified start block height or only unconfirmed ones if it is
set to -1.

lnrpc/walletrpc/walletkit.proto

@@ -1163,6 +1163,13 @@ message ListSweepsRequest {
     replaced-by-fee, so will not be included in this output.
     */
     bool verbose = 1;
+
+    /*
+    The start height to use when fetching sweeps. If not specified (0), the
+    result will start from the earliest sweep. If set to -1 the result will
+    only include unconfirmed sweeps (at the time of the call).
+    */
+    int32 start_height = 2;
 }
 
 message ListSweepsResponse {